In the various existing manufacturing methods, the hydrothermal treatment of the zeolite is always followed with an acid treatment. The hydrothermal treatments are well known in the art and provide so-called stabilized or even ultra-stabilized zeolites. Thus MAC DANIEL and MAHER have claimed in U.S. Pat. No. 3 293 192 the production of so-called ultrastable Y zeolites characterized by a crystalline parameter from 24.45.times.10.sup.-10 m to 24.2.times.10.sup.-10 m and by small sodium percentages, due to the association of hydrothermal treatments and cation exchanges with ammonium salts solutions, KERR et al. have also obtained Y zeolites of increased silica content by selective extraction of aluminum by means of a chelating agent such as tetraacetic ethylene diamine (U.S. Pat. No. 3,442,795).
EBERLY et al. have combined the two last techniques for obtaining dealuminated zeolites (U.S. Pat. No. 3,506,400 and U.S. Pat. No. 3,591,488). They show that the hydrothermal treatment consists of selectively extracting tetracoordinated aluminum from the aluminosilicate structure. They claim this procedure as well as the subsequent treatment with solutions containing different cations. One example consists in the subsequent extraction by 0.1N HCl, giving a faujacite no longer containing aluminum. (This example has however been later disputed by SCHERZER who could not obtain the so-described product (journal of Catalysis 54,285, 1978).
WARD describes the manufacture of zeolite catalysts intended for the manufacture of middle distillates (U.S. Pat. No. 3,853,742). The zeolite is stabilized but is not subjected to acid etching at the end of the series of treatments and its crystalline parameter ranges from 24.40 to 24.50.times.10.sup.-10 m. BEZMAN and RABO have used, as basic material for hydrocracking catalysts, more strongly stabilized zeolites whose crystalline parameter varies from 24.20 to 24.45.times.10.sup.-10 m (EP 0028938). This zeolite type is more particularly characterized by a ion exchange capacity "IEC" lower than 0.07. The ion exchange capacity IEC is defined in this patent as: ##EQU1##
k being the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio determined before the back exchange to Na.sup.+ ions. A zeolite of SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio equal to k and of IEC value equal to 0.07 corresponds to the approximate formula: EQU H.sub.0.93 Na.sub.0.07 AlO.sub.2 (SiO.sub.2).sub.k/2
The sodium ion uptake capacity in such a product is expressed, in % by weight, as: ##EQU2##
When k=4.8, C.sub.Na =0.78
When k=10, C.sub.Na =0.45
Hence, for an IEC value lower than or equal to 0.07, the retaking sodium ion capacity C.sub.Na is always lower than 0.8.
The zeolite ultra-stabilized by the BEZMAN and RABO method is also characterized by its hydrophobic character, such that its water adsorption capacity at 25.degree. C., at a P/P.sub.o value of 0.1, be lower than 5%.
SCHERZER (Journal of catalysis 54, 285, 1978) synthesizes zeolites of very increased silica content (SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio .gtoreq.100) by combination of hydrothermal and acid treatments, said zeolites being characterized by X-ray diffraction. At the same period, V. BOSACEK et al. also proceed to similar treatments for obtaining an ultra-stable zeolite having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 75.
These products are too strongly dealuminated and, for this reason, their interest in cracking operations is doubtful. As a matter of fact, it is necessary to maintain a minimum number of aluminum atoms in the structure in order to maintain a sufficient acidity required for a cracking catalyst.
The Belgian Pat. No. 895 873 indicates the possibility of producing middle distillates by hydrocracking with the use of a catalyst containing Y zeolites treated with stream and then lixiviated. This ultra-stabilized zeolite is characterized by different parameters, particularly a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio higher than 10, a crystalline parameter lower than 24.4.times.10.sup.-10 m, a particular mesopore distribution. The porosity of an Y zeolite, non treated with steam and with an acid, is entirely pores of a diameter lower than 20.times.10.sup.-10 m.
Ultra-stabilization treatments change this distribution. According to the Belgian Pat. No. 895 873, the described treatments generate a mesoporosity centered on about 80.times.10.sup.-10 m for a zeolite treated with steam and about 135.times.10.sup.-10 m for the same zeolite subsequently subjected to an acid treatment.